In my opinion you should take no pay cut whatsoever.

Consider first that you're probably underpaid to begin with, as are 90% of all Fox developers who don't realize the degree to which demand dwarfs supply.

Also, given that he'll likely have to pay more than what he's paying now to get a Delphi developer, and moreover you've got problem/domain expertise that no newcomer will have, and given the risk that the hired delphi developer could be, to some degree, a real dud (we're all duds to some degree), then that means he's getting a bargain at your current rate.

In the old days one would take learning into consideration. Today constant learning is a fact of life, and you'll do no more daily learning in Delphi than you do now in Fox. All the OO issues are still present, and the actual coding mechanics are rather easily assimilated in a short time.

You're crazy to sell yourself so short.