This is done through the creation of secondary wallets with unique private keys that enable the transfers of exact amounts. (sludgefeed.com)
Money belts keep your finances organized and all in one place, and I know plenty of travelers that bring a money belt along as a kind of secondary wallet. (blog.tortugabackpacks.com)
Paypal proposes to create secondary wallets with separate private keys for buyers and sellers and let the transaction happen between those wallets instantly before it is registered in the blockchain. (fxstreet.com)