Question about I-751 and travel to mexico?

My husband just filed to lift the conditions off his permanent residency. He got a letter stating that he can travel and work but did not get anything stamped in his passport when he went to get his fingerprinting done..we are travelling to mexico in two weeks so I just wanted to know if we are ok travelling with his passport, the letter of receipt, and his expired greencard? any answers would be appreciated!

This complicated...he needs to ask immigration. I cannot imagine WHY he would let his green card expire! I think he shoud definetly NOT leave the U.S. with an expired green card as he is very unlikely to be allowed back into the U.S. he needs to contact the office he got that receipt from and ASK.