Can I mandate in a contract that the client is responsible to pay the full amount of money guaranteed by it in on-site print sales?

Question Details:

The client does not want to pay a retainer but is willing to guarantee me $1200 in on-site print sales at the event. The client will be liable to pay the difference if I do not reach $1200 in on-site sales. Can I stipulate in the contract that the client is liable to pay me the $1200 if the event is cancelled or moved for any reason, including act of God?

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