Lowell explains the large amount of cash on his person by mentioning Charles Keating. Keating was made infamous for his involvement in the Savings and Loan (dubbed "S&L") scandal and resulting financial crisis of the late 1980s, where over 23,000 of Keatings customers alone were left with worthless bonds.
Tony Shalhoub returns to the series after being absent for the last two episodes of season 3, during which time, his character Antonio was visiting family in Italy.