Your municipality has the right to tax the earned income of its residents regardless of where earned.
Any unused "credit for taxes paid to another state" can be used as a credit on your Berkheimer local earned income tax return.
Say you paid $100 to DE. Your PA Schedule GL allows you a credit of $80 on your PA tax return. The remaining $20 difference can be used as a credit on your Berkheimer return. This credit can not be used to obtain a refund of local taxes.
See line 12 of the Berkheimer form https://www.hab-inc.com/wp-content/uploads/FinalWEBDCEDFPM.pdf
and line 12 instructions.
