There's nothing illegal about it. However, if father wishes to challenge the shared custody order, he could try and make an issue out of it. If he tries to modify custody based upon this, I would seek legal representation.
What kind of agreement? Was it an agreement through Domestic Relations, or a private agreement? There are certain 'support guidelines', which can be found in the Pennsylvania Rules of Civil Procedure book, that serve as a guideline as to how much child support you would be ordered to pay, should the two of you go through Domestic Relations. I would suggest you arrange a full consultation with an attorney to discuss your child support options, and the amount you would be expected to pay.