Can I be on the purchase and sale agreement if my husband is going to be the only one on the loan?

This probably isn't going to work.  The P&S has to go to the lender, as proof of what the purchase price is going to be, and the P&S is going to be read as stating who is going to be on the title to the property after the deal closes.

A mortgage lender usually can't live with having someone on the deed who isn't also on the mortgage, because that means that the lender doesn't have the full ownership of the house as its collateral;  if you were on the deed but not the mortgage, it would probably be much more difficult for the bank to get the property sold to pay the loan in the event of a foreclosure.