Can the seller legally require that he/she is paid with a conventional loan and not an FHA loan?

We recently signed a contract agreeing to purchase a house from a private seller. In the contract is specified that the seller is to be paid with a conventional loan. We were not approved for a conventional loan but were approved for an FHA loan. The seller won't accept the FHA loan and we must be out of our current house in two weeks. Can the seller legally require that he/she be paid with a conventional loan only? If not, should we take legal action? This has all put a huge strain on my family and I. We feel like we are being bullied. The seller is very difficult to communicate with.