This is a valid construction, but you are extremely unlikely to encounter it outside of a business context. It is very much at home in an accounting filing, though, as seems to be the case here. Sale of land refers to the total aggregate gain from all sales of land during the fiscal reporting period. The phrase refers to selling land as a revenue producing activity in general, and not to any particular sales.
As it stands, this sentence is ungrammatical. Land sale is singular and should be either preceded with an article or pluralized (or both). If there was only one sale during the period in question, use the land sale. Since there was only one possible sale, it's understood exactly what's being discounted from the total, so use the definite article. If there were multiple sales, use land sales.
Use this if multiple land sales occurred during the time frame in question. You may optionally add the before land sales. The difference? Including the makes it understood which sales are referenced (the ones that took place during the accounting period in question). Land sales without an article means any land sales. It is also correct, but less formal, to use land sales to mean the same as in 1, but when doing so, do not use any article at all.
In the context of the example, all three meanings have the same effect: the before-tax profit does not include revenues generated by selling land. If no land sales took place, it would be nonsensical to call this out as being excluded from gross profit.
I intend to say a singular land sale. But I understand that sales also means "gross receipts", so in (3) does it mean profit from many activities of land sales or from the gross receipt of a single land sale ?
Construction 1 uses the phrase in the sense of gross receipts from a particular activity, which many involve one, many, or even no individual sales. Version 2 means exactly one sale took place. Sentence 3 typically means that at least two sales occurred, though you can also use it in the sense of gross receipts, as in 1. In a technical, legal or fiduciary context, I would expect to encounter version 1 for this sense, though, because its meaning is unambiguous.
Because of the sentence structure and the fact that you are talking about a single sale only, I suggest using either 1 or 2, but not 3. 1 unambiguously means selling land as a general activity, and 2 specifies a single sale. 3 can be mistaken for meaning that multiple sales occurred. This also strikes me as being a more formal context, making 1 more appropriate than 3 if you want to talk about gross receipts.