"Say: 'Spend willingly or unwillingly'"—meaning: from what is imposed upon you regarding expenditures for Jihad.
﴿قُلْ أَنْفِقُوا طَوْعًا أَوْ كَرْهًا﴾ يَعْنِي: مِمَّا يُفْرَضُ عَلَيْكُمْ مِنَ النَّفَقَةِ فِي الْجِهَادِ