and how are you supose to end the current stage whitout starting the new one? you trade he take the blade, you talk to him he give you the new one back.... this starting the other stage?
For example, i started dagger, and traded the materials and currency to get stage 2. Next day i got back my dagger, and traded a horn with mats and currency, and next day, the horn stage 2 was complete.
For your problem, aparently, the gob doesnt have a weapon. So, i supose the weapon stage 3 is with you right? If thats the case, you can just trade the shield and mats/currency, and he will make the shield stage 2. But if your sword stage 3 is with the gob, there is no solution other than giving him the attestation and the 60 currency to complete.
The hint is to never give anything to gob if you cant finish it right away.