Use this to extract your save with homebrew launcher.
https://gbatemp.net/threads/save-data-manager-and-editor-for-firmware-up-to-9-9.396245/

Using a hex editor you can edit how many coins you want on your coin count. I have tested this with an EUR versions of the but not any others. I don't know the limit just yet but i think it could be 9999999. I might have some info wrong as im new to this stuff. Any one can use this information for documentation or making a program.

Video:


Files:
BigRedSave.dat (Edit)

Offsets to edit:
0x00001F10 - Coin Count Amount

Coin Amounts:
(Replace the Offset from above to one of these)
342150 Coins - 00 00 00 00 00 00 00 00 86 38 05 00 03 00 50 30