Barter? There are a number of sites that will trade services. If you have a product or service that you can offer, there may be a dentist who is looking to barter with you.
No. You should only replace fillings if they are breaking down or there are other problems with them. Removal of fillings often leads to removal of more tooth structure, so you need to be aware of this. All dentists have to pay rent, utility bills, costs of materials and staff salaries, so it is highly unlikely that you would find someone to replace your fillings for free.