I've never heard that 90% of northern slaves were sent south. Undoubtedly many were, as some slaveholders in the North desired to get something out of their financial investment. However, most northern states chose gradual emancipation as a way to avoid great financial shock to slaveowners in their states.

Historian Peter Kolchin briefly discusses the abolition process in the northern states in his _American Slavery, 1619-1877_. He says,

"During the three decades following the outbreak of the Revolutionary War, every Northern state initiated complete slave emancipation. The proces varied considerably. In some states, emancipation was immediate: the Vemront constitution of 1777 prohibited slavery, and soon thereafter Massachusetts courts, reacting to a series of freedom suits brought by blacks themselves, interpreted that state's constitution as outlawing slavery, to; as the state's chief justice put it in 1781, 'there can be no such thing as perpetual servitude of a rational creature.'" In most Northern states, however, especially those with a significant slave population, emancipation was gradual, so as to provide as little shock to society (and the masters' pocketbooks) as possible. According to Pennsylvania's law of 1780---the first of five gradual emancipation acts passed by Northern states---all future-born slaves would become free at age twenty-eight. New York's law of 1799 freed future-born boys at age twenty-eight and girls at twenty-five; New Jersey's act of 1804 (the last emancipation act of a Northern state) was similar, but provided that boys would receive freedom at age twenty-five and girls at twenty-one. Because these gradual emancipation laws freed no one actually in bondage at the time of their passage, and freed children subsequently born into slavery only when they reached adulthood, the North contained a small number of slaves well into the nineteenth century. By 1810, however, about three-quarters of all Northern blacks were free, and within a generation virtually all would be free." (Kolchin, 78-79)

He does not say, or estimate, the number of slaves sent South. One gets the impression from his work that the number was not anywhere close to 90%.

One possible way to estimate the number, though, is to compare census data.

For instance, here are the slave statistics of northern states in 1790:

Connecticut: 8,887
Massachusetts: 0
New Hampshire: 157
New Jersey: 11,423
New York: 21,193
Pennsylvania: 3,707
Rhode Island: 958
Vermont: 0

Now, in 1800 we can search slaves and "All other free persons" which, apparently, excludes free white persons.

So, here are the states, "All other free persons," and slaves

Connecticut: 5,330 -- 951
Massachusetts: 6,452 -- 0
New Hampshire: 852 -- 8
New Jersey: 4,402 -- 12,422
New York: 10,374 -- 20,613
Pennsylvania: 14,564 -- 1,706
Rhode Island: 3,304 -- 380
Vermont: 557 -- 0

In 1810, we can conduct a similar search.

States/"All other free persons"/slaves

Connecticut: 6,453 -- 310
Massachusetts: 6,737 -- 0
New Hampshire: 970 -- 0
New Jersey: 7,843 -- 10,851
New York: 25,333 -- 15,017
Pennsylvania: 22,492 -- 795
Rhode Island: 3,609 -- 108
Vermont: 750 -- 0

The census of 1820 formally recorded "Free colored persons." Here is the data from that census.

State/"Total Free Colored Persons"/slaves

Connecticut: 7,870 -- 97
Massachusetts: 6740 -- 0
New Hampshire: 786 -- 0
New Jersey: 12,460 -- 7,557
New York: 29,279 -- 10,088
Pennsylvania: 30,202 -- 211
Rhode Island: 3,554 -- 48
Vermont: 903 -- 0

(All of this data can be accessed at http://fisher.lib.virginia.edu/collections/stats/histcensus/)

Briefly looking over this information, we see a steady rise in the free black population as well as a steady decline in the slave population. New York had just over 10,000 free blacks in 1800, while recording around 20,000 slaves. In twenty years, the free black population of New York was almost 30,000 while the slave population was down to 10,000.

New Jersey's numbers also show a pretty steady progression. In 1800, NJ had almost 4,500 free blacks and almost 12,500 slaves. In 1810 those numbers had changed to roughly 8,000 free blacks and 11,000 slaves. In 1820, we find 12,500 free blacks and 7,500 slaves. That shows a loss of roughly 5000 slaves and a gain of 8000 free blacks.

Now, it is possible that a large percentage of slaves were sold south during this period from states like New Jersey. But that would suggest that the free black population was increasing either by a massive influx of freed slaves from other areas, or from a very high birth rate and low death rate. I believe the free black population rose from some combination of manumission and these other factors.

This information, of course, is only basic and cannot show exact numbers of slaves sold south or former northern slaves who were manumitted. But I believe that it suggests that a significant number of slaves secured their freedom and remained in the North.