Ever hear of tortuous interference? When one party induces another to break their contract with a third party. For as realistic as your example is (no employer is going to sue because you mowed your neighbors lawn), plenty of employers can, have and will sue when they realize you have hired away one of "their" people, regardless of your rights and theirs.

Learning of your little deal to pay them for software development work will be the stick they use to hit you with that interference suit.