a. Although not a yacht, I managed to buy a car.
b. I managed to buy a car, although not a yacht.
I think in theory there are two possible meanings:
I managed to buy a car instead of a yacht.
I managed to buy a car, but not a yacht as well.
Would you say that both meanings are possible both in the case of (a) and (b)?
Many thanks.
"Although" means despite the fact that, or despite being. It highlights a contrast between two things, rather than a relationship. It does not mean "instead" or "not as well". So, sadly, neither of your interpretations is correct.
A yacht is typically far more expensive than a car. If one only just "manages" (succeeds with some difficulty) to buy a car, they likely could not afford a yacht.
The implication with both sentences (although I find the first more idiomatic) is that the person would like to buy a yacht, and perhaps even previously stated that desire, but is taking consolation in the fact that they achieved something lesser in affording a car.
Consider:
Although I didn't win, I managed to come in second place.
The person wanted to win, but didn't; however, they came second, which is the next best thing. Obviously, one cannot come in first and second place, so this nicely demonstrates that they can be mutually exclusive.