Re: 80 Euros per day
That is considered an allowance and the thing is, you won't spend up to 80 Euros a day even if you add 3 meals together. 80 euros x 21 days = 1680 euros which is equivalent to SGD3360, assuming that he would be paid 80 Euros as well during weekends.
Re: No promotion in a small firm
That's regardless of the case. Even if it's a small firm and your husband may intend to job hop after, having been abroad would be something really nice to put in a resume especially since he's being sent to Europe and not an Asian country. (Well, I assume he's going to Europe since he's going to spend in Euros).
Moreover, since he has been travelling, I believe the company has expansion plans and since he has been with the company, he would also be in the forefront for promotion if that happens.
Re: Not trusting him being overseas
I believe marriage and love is based on trust. If you can't trust him being overseas because you're afraid he would be unfaithful, him being in Singapore wouldn't ease that fear any bit just because you are able to see him more often. I would encourage you to learn to trust your husband.
My husband flies every month as well. His longest stint was 3 weeks as well. Although when he tells me his schedule to fly, I will be very sian about it, we do go around making necessary arrangements so that it wouldn't be too hard on myself. For me, I live on my own with my husband and son. When my husband flies, I drive everywhere and do everything on my own with my son. Do I miss my husband and wish he is around on weekends to play with my son? Hell yes.
It's always a trade-off for what you want. I believe your husband can remain in Singapore and leave the flying to someone else or even change a job. Is that the career progression that he wants? Being based in Singapore could also possibly mean earning less. Is that what you want and what he wants?
I myself used to fly twice a month for about half a year before I got pregnant. I didn't even get a single cent from my company as daily expense. Sometimes I even pay for my own meals at my own expense and I am working for a company with staff strength of over 200 in Singapore alone.
My point is, you reap what you sow. If you want your hubby to stay in SG, dont't blame him later on for complaining that he hasn't progressed in rank and file for the last x years. If you dont't trust your husband, even late nights spent out would arouse your suspicions.
I'm sure he can understand things from your perspective. Try also to understand his perspective. Living together is all about working together and making a few compromises here and there for the marriage to grow. If everything stagnates, what growth is there to speak of?