The balance will remain on your card. I recommend that you register your card on the Marriott website (see wiki) and check the balance after any transaction. The exchange rate is very close to the XE rate so you should be able to calculate what the approximate balance should be. (I’ve had one instance where the front desk accidentally zeroed out my card, but they noticed it before I did and gave me a new card.)

In the past, I might have recommended smaller gift cards because of the chance of someone stealing your gift card balance, but the last time I logged in to the gift card website, I saw that Marriott introduced an option lo lock your gift card which is a huge improvement to prevent fraud. So I would prefer to have a larger value gift card now.

I’ve used gift cards on all continents (except Antartica) and rarely have issues. I recommend never waiting until the last minute to pay your bill because sometimes only the mgr knows how to process gift cards, or the accountant, or the guy who has the password, etc. If I’m checking out early in the morning, I pay my bill the night before.

Hope that helps!