need to make all payment with EGP

hello
i hope you can support me
my default site currency is USD but my payment gateway only support EGP
so i make the EGP is only the checkout currency for this payment getaway
but when customer place an order its give error ( failed to register the order )
and its showing the 2 currency at the checkout ( you will paid EGP XXX) which means the payment will be EGP not USD
i turned off the plugin and