But if anyone knows about slavery they should, don't you think?
Since they are the masters of slavery, I am sure they have the fewest credibility. I can't get into your logic. That's the same as asking a Cuban Communist to tell you about American healthcare.
Tell me what sources you would trust and I'll see what I can dig up.
Any scholastic, scientific proof. If you are keen on showing "they admit it" it would have to be from ancient Persian sources, or third ancient sources. Note, I am not denying there could have been slavery (paid or unpaid) in Persia. I only vehemently disagree with your methodology of proving it. Babylonian law doesn't apply to Persia, and a claim from an Islamist website doesn't equate an ancient Persian claim, as you insinuate.
Would you agree that Iran was once Sumer?
http://hsc.csu.edu.au/ancient_history/societies/near_east/persian_soc/persiansociety.html
Slavery
Domesticated animals and enslaved humans and a vast number of people were needed to work on projects of agriculture, warfare and monumental construction. State owned slaves in the mines (Olmstead, 1948: 74 ff), and they were well paid (Dandemaev and Lukonin, 1989: 161-2), but they had the status of livestock moveable property (op. cit 153). The household of the Great King maintained a large retinue of slaves who functioned as plowmen, millers, cow herds, shepherds, winemakers and beer brewers, cooks, bakers, wine waiters and eunuchs (Dandamaev and Lukonin, 1989: 158, 170). Of the slaves at Persepolis, 12.7% were boys, and 10% were girls (Fortification Tablets). Dandemaev and Lukonin (1989: 1601), concluded that these slaves lived together as families but they were also moved around the empire in what amounts to job lots. Documents record the movements of between 150 and 1500 slaves from one site to another. In Babylon, Egypt and the Greek cities of Lydia, the arrangements predating the Persians were kept. Slaves were usually acquired through warfare (Falcelière et al, 1970: 433), and were known as “the booty of the bow” (Dandamaev and Lukonin, 1989: 156). The peace established by the Great King would have effectively dried up this source. However, the Great Kings enslaved satrapies and cities which rebelled (Dandemaev and Lukonin, 1989: 170). Slavery was usually seen as a hereditary state, the children of those slaves maintained private stocks. Household slaves could be bought (Herodotos, vm, 1os). There was a privately owned slave labour force doing menial tasks. In Babylon, debtors could sell themselves into slavery (Olmstead, 1948: 74 ff), but this quickly died out under Persian rule (Dandemaev and Lukonin, 1989: 156). Everyone from the highest nobles down were defined as bandaka (the slaves of the Great King) (Kurht, 1995: 687), or ‘those who wear the belt of dependence’ (Wiesehoefer, 1996:31). This meant that taxation was due in money, precious metals, goods, military service and labour.