How can you go to a mechanic and say you want a BRAND SPANKIN' NEW A/C system.... and then in the same breath say you are low on funds? From what you said.. it looks like the man wanted to help you out and saved you cash by using parts that are working... i highly doubt he would knowingly or purposefully cheat you by putting parts in that are about to go... especially if he has been doing work on your vehicles before.

Do what you said you will.. take it to another pro who can verify whats wrong. If it turns out to be something wrong with his worksmanship... you can settle this with him. If it turns out to be a part that he put in that went bad... settle it with him... maybe he will replace the part.