During the constitutional convention the commerce and slave trade compromise gave congress the power to

Regulate commerce in both the North and the South, but could not yet regulate the slave trade.

Explanation:

The constitutional convention was made to write a new constitution for the USA, in that convention were solved many problems, among these problems, the guidelines between commerce and the slave trade were resolved.  In this regard it was decided that the Congress could not prevent the slave trade, but a tax would be levied on each imported slave.