I don't think you have enough information here to make a determination. But you do have a sufficient need to ask more questions.
What do you know about the customer, the doughnut shop, the age/condition/appearance of the currency, the normal and expected cash use in your customer's region, any relevant demographic factors (age, national origin, etc.) that might explain the customer's inclination toward storing cash in the ground?
Once you have a complete picture of your customer, what is normal and expected, and what actually happened, you'll be in a good position to make a determination as to the SARworthiness of this transaction.