They may not have MFK marketplace in Saudi Arabia.Make a listing on mfk marketplace or something else so someone with a good home can find it, if your cousin? Likes the look of pacu, silver dollars look like them but are way smaller
For now:
Try adding some plastic plants and dim the light so the pacu feels more comfortable. It seems like something is scaring your cousin’s fish so it’s darting into the glass. Is the aquarium in a busy location where lots of people walk by?
Soon:
It would be best to transfer the pacu to the pond. Keep the pond out of sunlight, don’t overfeed, add a filter, and do water changes
Later:
You need to rehome it to someone or somewhere where it can have a larger home. Don’t release it into the wild