New motherboard and used processor

so i fried my motherboard, got it fixed and he put a single core procssor in it. this isnt nearly fast enough, so i want to put my processor from my old computer in this one, which is a dual core. i talked to the guy at the computer shop, and he said that using a new motherboard with a used processor isnt good? like they are not compatible or something like that? i didnt think this is true at all and was wondering if it is true or not?

Find the model number of the new board and see if the old CPU will match the allowed CPU's for that model board

the socket on the cpu has to be the same as the motherboards socket, it will most likely be socket 775 or AM2 but if they don't match, poof goes your hardware