(a) Under section 7(g)(2) an employee who performs two or more
different kinds of work, for which different
straight time hourly rates are established, may agree with his employer
in advance of the performance of the work that he will be paid during
overtime hours at a rate not less than one and one-half times the hourly
nonovertime rate established for the type of work he is performing
during such overtime hours.
Don;t know if there is anything stating the employer must agree to that. It seems as if there is no requirement of such. Maybe cbg could address that. This is not my "thing". She is much more knowledgable on the subject.
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eventhorizon, try looking at things a bit differently.

the reason the higher wage work was overtime was because you performed many hours of the lesser wage. Is it fair to the employer to pay you 1 1/2 time the higher wage when it was the lower wage hours that allowed/caused it to be overtime to begin with.

It sounds simply like you should opt to take the ot hours at the actual wage assigned to that job and give up. While arguing for overtime with your employer (apparently incorrectly as things look) cannot be responded to with termination, the fact you wore a green shirt on Monday could be