Except such a plan was tried - at Île à Vache - and that pilot project failed.
Lincoln would have no legal right to force freed slaves to leave the US, and as Frederick Douglass and others communicated to Lincoln such a plan to voluntarily leave was not well received by black slaves. The majority had been born in the US and several generations had passed since their forebears had been forcibly brought to the US.
“Lincoln would have no legal right to force freed slaves to leave the US”
Laws can be changed. In fact that period saw some of the most consequential new laws passed including constitutional amendments, which is a very difficult thing to do.