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True that. The legal principal is called "quid pro quo".
Wiki: Quid pro quo (From the Latin meaning "something for something") indicates a more-or-less equal exchange or substitution of goods or services. English speakers often use the term to mean "a favor for a favor"...
...If the exchange appears excessively one sided, courts in some jurisdictions may question whether a quid pro quo did actually exist and the contract may be void by law.