The salary is defined as income in cash and/or in kind received by the natural resident or non-resident on the basis of an individual employment contract, an employment relationship, a posting contract or a special law provided for by law, and is taxed at a flat rate of 10%. Annual gross rental income, with the exception of the land lease, represents the income received by the owner for a given year, as stipulated in the lease entered into between the parties. . . .