The exchange rate of the Dollar to the Cedi in Ghanaian forex bureaus apparently depends on the denomination, that is to say, the value on the face of the note.
For the closest rate to the prevailing international rate, a note has to be at least $50. There are different rate s for $20 and $5 notes too. The $1 note, when they accept it, usually is converted to the cedi on a 1 to 1 basis.
This practice seems to be widespread; it’s so in all the forex bureaus I have visited. As much as possible, have higher denominations if you plan to exchange your Dollar to the Ghana Cedi. You might also want to try banks.