Moshimo Kubiwa ga Hazuseta Naraba

"I bought an old slave with one coin (and got change back)." Tanya, a lonely girl, buys a slave, William, for one coin. Tanya wants to be family, but they're "master" and "slave." It's the incongruously woven daily life of two people with different ages and social standings.
(Source: Comic Polaris, translated)