Negative spending cause addition

[nus-se-script]

Negative spending in reminder caused the savings to increase rather than decrease.

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_{[original: nus-cs2113-AY2324S2/pe-interim#2152] [original labels: type.FunctionalityBug severity.High]}

[chenhowy]

Team's Response

Accepted.
Bug is caused by improper handling of positive and negative inputs.

Duplicate status (if any):

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