1. I wish to contract a gentile moving company whom a frum, Jewish contractor found for me
2. If I pay the moving company directly after they finish, they cost $ x
3. I asked the contractor if they can wait to get paid a week later after I close on my house and have the funds
4. The contractor said that he wants $300 more because he would have to take a gemach to pay the gentile moving company for me
1.. Is the frum, Jewish contractor allowed to charge me a fee for paying the gentile moving company?
2. Is his payment to the moving company loan to me?
3. Does his charging me a fee for paying the gentile moving company considered to as ribis?
Yes, his paying the workers for you would constitute a loan, and hence charging interest would be a violation of ribbis. While he could set a price for providing workers, in your case you have the option of paying them directly and he is explicitly charging you for extending the time for payment.
Rama Y:D 161:1, Taz 2