My first thought was that it meant usury, but apparently Buddhagosa defines it as,
'Herein, what is pursuing gain with gain? Seeking, seeking for,
seeking out, going in search of, searching for, searching out material
goods by means of material goods, such as carrying there goods that
have been got from here, or carrying here goods that have been got from
there, by one bent on gain, honour and renown, by one of evil wishes,
a prey to wishes—this is called pursuing gain with gain' (Vbh. 352-
... which I think implies being a merchant (buying here and selling there), or more generally capitalism.
Note that that sutta is addressed to monks.