Is offering a fixed payment for every successful hit in a set number of attempts valid?

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It is valid if the offeror states, 'If the majority of your shots are hits, you will receive one dirham for every arrow you hit with,' or 'Shoot ten arrows, and you will have one dirham for every arrow you hit,' or 'You will have one dirham for every arrow that exceeds half of the successful shots.' These are valid because the wage is determinable by the success rate, analogous to offering a date for every bucket of water drawn from a well, or offering one dirham for every slave returned.