No, you do not have to pay it back.
The lump sum benefit you received is known as the Basic Employee Death Benefit. It is equal to half of your spouse's final salary plus an additional amount. Surviving spouses choose whether they want to receive this benefit in a single payment or in equal installments over 36 months. This benefit is only available under FERS.
FERCCA has special rules for surviving spouses who choose CSRS Offset rather than FERS and were paid a Basic Employee Death Benefit. Instead of you paying it back, OPM will apply an actuarial reduction when it calculates your CSRS Offset survivor benefit.
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