
If you had already negotiated with the teen for $10.00 per day, then
it is unreasonable of you to change the terms of your agreement just because you changed the feeding schedule of your animals.
You offered $10.00 per day, the teen agreed to do it for that price.
It is not fair for you to change the terms of the deal.
And not only that, but word will get out that you don't stick to agreements and you will have a much harder time next time finding a teen willing to do work for you.
Is it really worth it?