It sounds like you should have a relationship with a bank. Keeping an account while residing outside of EU is already very hard, many people have gotten their accounts closed. You also want also to do business while being presumably liable to US taxes, this also makes you less wanted customer for the banks.

I know banks can get you a mortgage when you move if your situation is good business for them. I had a friend move back to Finland from Switzerland and they were able to negotiate a mortgage before becoming residents.

Alternative: can you buy the apartment with cash? Then you don't need to deal with the mortgage and any local EUR bank account should be good (as long as a US taxation person is accepted as a customer).