The developers changed what happens when you drop an ally.
You must now own an ally for a minimum of 15 days to receive the maximum refund value of 60% of their hire value.
Dropping an ally before owning them for 15 days is scaled based on how long you've owned the ally, with a minimum of 4.5% refund if dropped on the day you hire them.
Assuming a linear scaling, you will get an additional 3.96428571429% per day, starting on day 2 of ownership.
Example:
Hire an ally worth 1,000,000 gold. Drop on day 7.
Day 1 = 4.5% refund = 1,000,000 x 0.045 = 45,000 gold
Day 2 = 4.5% + 3.96428571429% = 7.96428571429%. So, 1,000,000 x 8.46428571429% = 84,643 gold.
Day 3 = 8.46428571429% + 3.96428571429% = 12.4285714286%. So, 1,000,000 x 12.4285714286% = 124,286 gold.
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Day 7 = 1,000,000 x 24.3214285714% = 243,214 gold.
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Day 15 = 1,000,000 x 60% = 600,000 gold.