Love Deep as the Sea by the Rhine

Amnesiac Ethan, a belittled live-in son-in-law of the wealthy Lowell clan, regains his mighty power and buys a luxury jewelry brand for his beloved wife Lorelei. When jealous rival Jasper schemes to force Lorelei into marriage, the pair team up to overcome constant sabotage, beat Jasper at a major auction, and claim their well-deserved happy ending after Lorelei wagers to raise a billion dollars in a month.