Conditions for a Sahih Li Ghairih hadith

I was looking on [sunnah.com](http://sunnah.com/abudawud/24/51) and came across this hadith: > عَنْ عَبْدِ اللَّهِ بْنِ أَبِي أَوْفَى الأَسْلَمِيِّ، قَالَ غَزَوْنَا مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم الشَّامَ فَكَانَ يَأْتِينَا أَنْبَاطٌ مِنْ أَنْبَاطِ الشَّامِ فَنُسْلِفُهُمْ فِي الْبُرِّ وَالزَّيْتِ سِعْرًا مَعْلُومًا وَأَجَلاً مَعْلُومًا فَقِيلَ لَهُ مِمَّنْ لَهُ ذَلِكَ قَالَ مَا كُنَّا نَسْأَلُهُمْ ‏.‏ > > *We made a journey to Syria on an expedition along with the Messenger of Allah (ﷺ). The Nabateans of Syria came to us and we paid in advance to them (in a salam contract) in wheat and olive oil at a specified rate and for a specified time. He asked (by the people): you might have contracted with him who had these things in his possession? He replied: We did not ask them.* It said that the hadith was **sahih li ghairih**. What is a **sahih li ghairih** hadith and what are the conditions for a muhaddith to declare a hadith **sahih li ghairih**?