I've been trying to send iron to a league member, but when I do, whatever amount I send changes to 5540 as the caravans are in transit. First time I've tried this since I upgraded my market place with sketches.
There is a limit on how much a castle can receive free resources, 50k I think. So if your friend has received 44460 iron that day if you send more than 5540 it will change to that. The limit will reset on 00:00 UTC.
Oh, btw. The limit for each resources are separated so even if your friend has received 50k iron he can still receive 50k gold.