The Beraisa is referring to a case where Levi did not ask Shimon to work on his field. Nevertheless Levi must pay for the benefit he received. However, since we are dealing with a field which must be worked on anyway, it is reasonable to assume that if Shimon would not have commissioned out this work, Levi would have hired someone else to do the job. Levi must pay because we assume that it does not really make a difference to him who did the work. On the other hand, if there is good evidence to believe that Levi really did not want the work done, he would not have to pay against his wishes.

I will try and explain the above in somewhat more detail. This is based on the Gemara below Bava Metzia 101a which discusses someone who went into another person's field and planted trees there without permission. The Gemara makes a distinction between a field in which trees are generally planted and between a field in which trees are generally not planted. If trees are generally planted in the field then the owner must pay the normal price for the work. This is because he would have had to hire workers anyway to do this work.

The Gemara there relates an incident where somebody planted trees in a field without permission. Rav said that the owner must pay the regular rate for the work done. The owner of the field protested that he did not want this type of planting done. Rav answered that if this the case, then the owner will only have to pay a lower rate. Since it now appears that this is a field in which trees are not generally planted, the owner will have to pay the lower of: the "Hotza'ah" (the expenses paid by the worker) or the "Shevach" (the profit accrued from his work). The end of the story was that the owner of the field put up a fence and started guarding the trees. Rav said that since he thereby showed that in reality he was happy with the trees planted, he therefore must pay the regular price for planting trees. This is because the field had now become one in which it is usual to plant trees.

Now to your very interesting question about the person who got back from vacation and discovered that somebody had done some beautiful but unsolicited improvements to his house. It seems to me that this would depend on a few factors. If the house was in need of renovations and the owner was accustomed to renovating in such a way, and if the renovators did a competent job then according to what we have learned above it seems logical that the homeowner should pay the going rate for the work that he received.

If these conditions were not met then the homeowner would not have to pay full price for the renovations. However, if shortly after finding such nice work invested into his house he sold his house for a good price and it was clear that he made a profit from the renovations done in his absence then he would have to pay full price for the work. This would be comparable to the case in the Gemara in which his actions proved that he was glad that the trees had been planted in his field and was subsequently obligated to pay full price.

Additionally, if it is clear that the owner does not want the improvements at all then this is comparable to the Gemara at the bottom of 101a which discusses someone who went into another person's ruin and built there without permission. The conclusion there is that the owner can tell the builder to dismantle the work he did and the owner would not have to pay anything (see Rif - 58b in Rif pages).

Dovid Bloom