Belle is breaking off their engagement contract - He's idolising his money instead of Belle.
Metaphor- money - 'do not worship false idols' (part of the ten commandments) as the only God for Scrooge is Belle.
Scrooge is worshipping the wrong idol and is corrupt by money.
"I have seen your nobler aspirations fall off one by one until the master-passion, Gain engrosses you."
"master" - adjective - Suggests that money is ruling over Scrooge and is controlling him. Like a master, Scrooge is almost a slave to money.
"Gain" - capitalised - personifying Gain - becoming the person Scrooge is interested in and almost dehumanising him. He's more interested in money more than he is in Belle.