A former member of the A's farm system, Cust came into the year with a career .285 average over 1,089 games with 240 doubles, 191 homers and 679 RBIs.
In 70 games at the major league level, spanning 144 at bats, Cust has a .222 average with nine doubles, five homers and 19 RBIs.
Cust led the minor leagues in walks last year with 143 in 138 games for Portland and collected 30 homers and 97 runs scored.
He is expected to start for Oakland this weekend.
There are two rules to a PTBNL transaction: The deal must close within a six-month timeframe following the conclusion of the rest of the trade, and the player must change leagues. Trading minor league players circumvents the latter part of the rule.