Is a non-compete valid if the company backs out of Agreed terms?

I worked for a GA company for 8 yrs an upon returning 14 mths later they wanted me sign a non-compete. We agreed upon expenses,commission an a monthly salary. Expenses to be submitted by 1st of month to be paid by 5th, commission to be submitted by the 5th to be paid by the 15th of the month an Salary to be pd on last day of month. I was to have a office no later than 6 months an I am currently still working out of my home. The non-compete is for the laws of SC an I work in GA. We have no location in SC- is this non-compete valid? and can the employer modify the agreement that was made prior to employment 3 years later? I haven't rec'd anything in writting (they are now refusing to pay commission that has been submitted an owe 7 months of commission.)At what point does the employer breach a non-compete?