During the plagues, a number of times Pharoah acquiesced to letting the people leave Egypt due to the harshness of the plague. He would call in Moshe and say "Fine, I will let you go pray to your God" or "I will let you go free" then he would say "Just pray to get rid of the plague".
Moshe would then beseech Hashem to remove the plague. Subsequently, Pharoah would rescind his initial agreement.
I see 2 lessons in this:
1. An agreement arrived at under duress is easily reversed. Aftyer Pharoah rescinded we do not find Moshe goign to protest and saying but you agreed, etc. Pharoah rescinded claiming he was forced to agree, and there was nothing to protest.
2. Never pay in advance of receiving the goods and services. Moshe would pray, the plague would be removed, and Pharoah would not let them out. Moshe should have said "I will pray for you when we are two days away", but he prayed first and then relied on Pharoah's goodwill to keep his word, which he never did.