Distinction between sale and contract agreement

In the lranian Civil Law, like French law, contract agreement is one kind of hire or personal employement. But there are some contracts that their nature is near sale. Sale is a contract for transfering the possession of objects, but transfering the possession of interest is subject to hire agreements. In contracts that contractor undertakes to work, also presents materials, therefore we confuse between the nature of sale and contract agreements. For distinction between these two agrements, authors suggest several criteri: 1. Some scholars put this contract in sale category, 2. Some, accept complex contract in this case, 3.The other believe that this contract is always Contract agreement and 4.The last solution is consideration of the intention of client from agreemts or, 5.Consideration of the economical value of the work and materials or finding primal engagement. Each criterion has some limitations. It seems in the first step that we must distinguish the primal obligation and if we couldnt find it, must define the contract as an inspesific contract.